I've seen this question come up a few times. I think it would be a good addition to the documentation or the wiki to describe the possibilities more explicitly. I was of the opinion to that they could be anywhere on the classpath, but accoriding to some posts it does not (always?) seem to be that way.
afaik now, the classes have to be process local, so in the par. But it could be I'm wrong. There are some posts in this forum on this subject. So search it. I'll file a jira issue to address this.